A new class of optical resonators is comprised of convex dielectric bodies which are substantially deformed from cylindrical or spherical symmetry. See: J. U. Nockel, A. D. Stone and R. K. Chang, Optics Letters 19, 1693 (1994); A. Mekis, J. U. Nockel, G. Chen, A. D. Stone and R. K. Chang, Phys. Rev. Lett. 75, 2682 (1995); J. U. Nockel and A. D. Stone, in Optical Processes in Microcavities, editted by R. K. Chang and A. J. Campillo (World Scientific Publishers, 1996). Such asymmetric resonant cavities (henceforth ARCs) exhibit high-Q (quality factor) whispering gallery (WG) modes (Q&gt;1000) at distortions as large as 50% of the undeformed radius, R, of the corresponding circular (symmetric) resonant cavity. The emission pattern from these modes is highly directional, in contrast to the isotropic emission from symmetric cavities.
WG modes of symmetric cavities (dielectric spheres and cylinders) have high Q (long lifetime) because the light trapped in such a mode (when described by ray optics) always impinges on the boundary at the same (conserved) angle of incidence, .chi., where sin .chi.&gt;=1/n (n is the index of refraction of the dielectric); hence the light is almost totally internally reflected. Due to the curvature of the surface, there is an exponentially small correction to the law of total internal reflection which allows light to escape after very long times (this is called "evanescent leakage"). In ARCs on the other hand, the dominant mechanism for emission of light is not evanescent leakage but direct refractive escape via Snell's law because the angle of incidence sin .chi. is not conserved. In the ray-optics language these ARC resonances correspond to ray trajectories which initially are in WG orbits with sin .chi.&gt;1/n, but after a large number of reflections with the boundary eventually impinge on it with sin .chi.&lt;1/n and are then directly emitted according to Snell's law of refraction. The high-intensity regions in the near-field correspond to the regions on the boundary of the ARC where most of the refractive escape occurs; the far-field directionality can be determined by following the refracted rays.
In FIG. 1, a prior art cylindrical resonator of radius R is shown. The motion of a light ray in a WG mode circulating around its cross-section is shown in FIG. 2a. The motion forms a regular pattern with the angle of incidence the same at each reflection. As noted above, since this angle is initially above the angle required for refractive escape and remains so indefinitely, escape of optical energy occurs isotropically (equally in all directions) by the exponentially slow process of evanescent leakage (this escape is not shown in the figure).
Nockel, Stone and Chang, in the paper entitled "Q spoiling and directionality in deformed ring cavities", introduced the idea that a deformation of the cross-section might induce directional emission due to refractive escape. The deformation they considered, shown in FIG. 2b, causes irregular (chaotic) ray motion and leads to refractive escape at a point 10 (FIG. 2b) with the far-field high emission directions shown in FIG. 2c. Note that the high emission directions are not parallel and hence intersect in the near-field leading to interference effects not describable by ray-optics. Moreover there are secondary (split) peaks at angles near those of the largest peaks. The near-field behavior of ARCs was not known at that time due to the limitations of the ray model used.
The occurrence of multiple and non-parallel emitted beams make the prior art deformed resonator unsuitable for optical devices employing ARCs. Moreover the lack of information about the near-field radiation pattern of ARCs made it impossible to design input and output couplers for this resonator.